Adsorptive separation of para-xylene with high boiling desorbents

ABSTRACT

The cost of separating para-xylene from other xylene isomers and C 9  aromatics by adsorption on zeolitic molecular sieves is reduced by the use of certain relatively high boiling desorbents including 1,4 diisopropylbenzene. Preferably the adsorbent is an X zeolite adsorbent containing barium or both barium and potassium ions at exchangeable cationic sites. The para-xylene components are selectively adsorbed onto the adsorbent. The non-adsorbed feed is then removed from the adsorbent and the para-xylene recovered by desorption. Any C 9  aromatic hydrocarbons and the other xylene isomers in the raffinate can be separated from these heavy desorbents by fractionation of the raffinate and the desorbent can be recycled to the process.

FIELD OF THE INVENTION

The subject invention relates to a process for the adsorptive separationof aromatic hydrocarbons. More specifically, the invention relates to aprocess for separating para-xylene from a feed mixture comprising atleast two xylene isomers, including the para-isomer, which processemploys a zeolitic adsorbent and a particular desorbent. The inventionis particularly advantageous in a process in which the feed contains C₉aromatic hydrocarbons in addition to the xylene isomers.

BACKGROUND OF THE INVENTION

The polyester fabrics which are in wide use today are produced fromparaphthalic acid which is in turn produced by the oxidation of paraxylene. Para xylene is typically derived from a xylene isomerizationzone or from a stream separated from an aromative rich precursor, suchas a C₈ aromatic hydrocarbon fraction derived from a catalytic reformateby liquid-liquid extraction and fractional distillation. The para xyleneis commercially separated from a paraxylene-containing feed stream,usually containing all three xylene isomers, by either crystallizationor adsorptive separation or a combination of these two techniques.Adsorptive separation is the newer technique which has captured thegreat majority of the market share of newly constructed plants for theproduction of paraxylene.

RELATED ART

U.S. Pat. No. 3,686,342 issued to R. W. Neuzil discloses thatparadiethylbenzene (DEB) is the a better desorbent for the separation ofpara xylene from mixed xylenes using an aluminosilicate adsorbent. Theuse of this desorbent is described as allowing a more efficientseparation with a higher purity extract stream being recovered from theprocess.

U.S. Pat. No. 4,886,930 disclosed the use of "heavy" desorbentscomprising tetralin or tetralin derivatives for separating para-xylenewhen the feed mixtures contain higher boiling aromatic hydrocarbons,such as C₉ aromatics, C₁₀ aromatics, etc. However, these materials arenot readily available except when synthetically produced. Therefore, itis desirable to find a higher boiling point material, i.e., "heavydesorbent", that meets the selectivity requirements for desorbents whichcan be used with feed mixtures containing C₉ aromatics and is availablefrom natural, rather than synthetic, sources, such as coal tardistillates, etc.

U.S. Pat. No. 5,012,038 issued to H. A. Zinnen addressed the issue of aheavy desorbent for para xylene recovery and disclosed the use of one ora mixture of diethyltoluene isomers, with the 2,3; 2,5 and 2,6 isomersbeing preferred.

In a similar manner U.S. Pat. No. 5,057,643, also issued to H. A.Zinnen, suggests the use of tetralin or alkyl tetralins as a heavydesorbent for the separation of para xylene from xylene/C₉ aromatichydrocarbon mixtures.

Indan and alkyl indane derivatives were proposed as heavy desorbents foruse in the adsorptive separation of para xylene with x zeolites in U.S.Pat. No. 5,159,131 issued to H. A. Zinnen.

SUMMARY OF THE INVENTION

The invention is the discovery of new "heavy" desorbents for use in achromatographic process for separating p-xylene from a feed mixturecomprising p-xylene, one or more additional xylene isomers (includingethylbenzene) and C₉ aromatic hydrocarbons. One broad embodiment of theinvention may be characterized as a process for separating paraxylenefrom a feed mixture containing paraxylene and at least one other xyleneisomer, ethylbenzene and C₉ hydrocarbons comprising contacting said feedmixture with an X or Y zeolite having metal cations at exchangeablecationic sites to effect the selective adsorption of said p-xylene andproducing a raffinate comprising the other xylene isomers, ethylbenzene,and C₉ aromatics; and, recovering p-xylene from said adsorbent bycontacting the resulting para-xylene loaded adsorbent with a heavydesorbent chosen from the group consisting of diphenylmethane, 1,4diisopropylbenzene and 1,3 disiopropylbenzene. These desorbents arehigher boiling than the C₉ aromatics, making it possible to separate theC₉ aromatics from the desorbent by fractionation so that the desorbentcan be reused in the process without building up C₉ aromatics in therecycled desorbent.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

In numerous processes described in the patent literature variouszeolitic adsorbents are used to separate the para isomer of dialkylsubstituted monocyclic aromatics from the other isomers. The separationof para-xylene from other xylene isomers is both widely described andwidely practiced. In these processes a feed stream is contacted with aselective adsorbent which adsorbs the desired isomer. The desired isomeris then removed from the adsorbent by contacting it with a desorbent,forming an extract stream. Benzene, toluene, and p-diethylbenzene weredescribed as suitable desorbents in the early references, withp-diethylbenzene (p-DEB) having become a commercial standard for thisseparation. P-DEB is a "heavy" desorbent (higher boiling than p-xylene)which allows for easier recovery of the desorbent from the extract andraffinate by fractional distillation. Unfortunately p-DEB has adisadvantage when the feed stream contains C₉ aromatics, as is often thecase when the feedstock is derived from a crystallizer. This problem isthe result of the boiling point of p-DEB being very close to the boilingpoint of C₉ aromatics in the feed. Because the C₉ aromatics aredifficult to separate from p-DEB by simple fractionation, the C₉aromatics gradually build up in the desorbent, which must be reused inany economically feasible commercial process.

It has been therefore been necessary in the commercial processes foradsorptive separation of p-xylene from feed mixtures containing theother xylene isomers to reduce the C₉ aromatics content of the feedstream to less than about 0.1% prior to passing the feed stream into theadsorptive separation zone. This is usually done by distillation in aso-called xylene splitter column. The substantial costs associated withthis practice, such as the capital costs of the xylene splitter and theutilities cost of the fuel and energy necessary to achieve substantiallycomplete removal of the C₉ aromatics, could be eliminated if it were notnecessary to first remove the C₉ aromatics. It is an objective of thesubject invention to provide a "heavier" desorbent for para isomeradsorptive separation processes. It is a specific objective of thesubject invention to provide a process which allows the adsorptiverecovery of para xylene from a feed stream comprising C₉ aromatichydrocarbons without requiring extensive prefractionation of the feedstream.

I have discovered a novel process for the adsorptive separation ofp-xylene from its isomers employing a zeolitic adsorbent and novel heavydesorbent(s) suitable for use when the xylene feed mixture contains C₉aromatic impurities.

During the adsorption step of the process a feed mixture containing amixture of xylene isomers, preferably including para-xylene, and C₉aromatics is contacted with the adsorbent at adsorption conditions andthe desired xylene is selectively adsorbed and retained by the adsorbentwhile the other components are relatively unabsorbed. The adsorbentcontaining the more selectively adsorbed desired xylene is referred toas a "rich" adsorbent--rich in the more selectively adsorbed xylene. Theunabsorbed raffinate components of the feed mixture are then removedfrom the interstitial void spaces between the particles of adsorbent andfrom the surface of the adsorbent. The adsorbed xylene is then recoveredfrom the rich adsorbent by contacting the rich adsorbent with a streamcomprising a desorbent material at desorption conditions. The desorbentdisplaces the desired xylene to form an extract stream which istransferred to a fractionation zone for recovery of the desired xylene.

The invention herein can be practiced in fixed or moving adsorbent bedsystems, but the preferred system for this separation is acountercurrent simulated moving bed system, such as described inBroughton U.S. Pat. No. 2,985,589, incorporated herein by reference forits teaching in the practice of simulated moving bed adsorptiveseparation processes. Cyclic advancement of the input and output streamsof this simulation can be accomplished by a manifolding system or byrotary disc valves as shown in U.S. Pat. Nos. 3,040,777 and 3,422,848.Equipment utilizing these principles can vary in size from the pilotplant scale shown in deRosset U.S. Pat. No. 3,706,812 to commercialscale, with flow rates ranging from a few cc per hour to many thousandsof gallons per hour. The invention may also be practiced in a cocurrent,pulsed batch process, like that described in U.S. Pat. No. 4,159,284 orin a cocurrent, continuous process, like that disclosed in Gerhold U.S.Pat. Nos. 4,402,832 and 4,478,721.

The functions and properties of adsorbents and desorbents in thechromatographic separation of liquid components are well-known, but forreference thereto, Zinnen et al. U.S. Pat. No. 4,642,397 is incorporatedherein.

Adsorbents to be used in the process of this invention comprise thespecific crystalline aluminosilicates molecular sieves classified as Xand Y zeolites. X zeolites, specifically X zeolites exchanged withbarium or barium and potassium ions at their exchangeable sites, are thepreferred adsorbents. These zeolites have known cage structures in whichthe alumina and silica tetrahedra are intimately connected in an openthree-dimensional network to form cage-like structures with window-likepores. The tetrahedra are cross-linked by the sharing of oxygen atomswith spaces between the tetrahedra occupied by water molecules prior topartial or total dehydration of this zeolite. The dehydration of thezeolite results in crystals interlaced with cells having moleculardimensions and thus, the crystalline aluminosilicates are often referredto as "molecular sieves" when the separation which they effect isdependent essentially upon differences between the sizes of the feedmolecules as, for instance, when smaller normal paraffin molecules areseparated from larger isoparaffin molecules by using a particularmolecular sieve. In the process of this invention, however, the term"molecular sieves", although widely used, is not strictly suitable sincethe separation of specific aromatic isomers is apparently highlydependent on differences in electrochemical attraction of the differentisomers and the adsorbent rather than on pure physical size differencesin the isomer molecules.

In a hydrated form, the type X aluminosilicate zeolites are representedby the formula below in terms of moles of oxides:

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :(2.5±0.5)SiO.sub.2 :yH.sub.2 O

where "M" is a cation having a valence of not more than 3 which balancesthe electrovalence of the tetrahedra and is generally referred to as anexchangeable cationic site, "n" represents the valence of the cation,and "y", which represents the moles of water, is a value up to about 9depending upon the identity of "M" and the degree of hydration of thecrystal. As noted from Formula 1, the SiO₂ /Al₂ O₃ mole ratio is2.5±0.5. The cation "M" may be monovalent, divalent or trivalent cationsor mixtures thereof. In the separation of the invention, "M" is bariumor a mixture of barium and potassium.

Crystalline aluminosilicates or zeolites are used in adsorptionseparations in the form of particle agglomerates having high physicalstrength and attrition resistance. The agglomerates used in separativeprocesses contain the crystalline material dispersed in an amorphous,inorganic matrix or binder, having channels and cavities therein whichenable liquid access to the crystalline material. Methods for formingthe crystalline powders into such agglomerates include the addition ofan inorganic binder, generally a clay comprising a silicon dioxide andaluminum oxide, to the high purity zeolite powder in wet mixture. Thebinder aids in forming or agglomerating the crystalline particles whichotherwise would comprise a fine powder. The blended clay-zeolite mixtureis extruded into cylindrical pellets or formed into beads which aresubsequently calcined in order to convert the clay to an amorphousbinder of considerable mechanical strength. The adsorbent particles maythus be in the form of extrudates, tablets, macrospheres or granuleshaving a desired particle range, preferably from about 16 to about 60mesh (Standard U.S. Mesh) (1.9 mm to 250 microns). Clays of the kaolintype, water permeable organic polymers or silica are generally used asbinders.

Those skilled in the art will appreciate that the performance of anadsorbent is greatly influenced by a number of factors not related toits composition such as operating conditions, feed stream composition,water content and desorbent composition. The optimum adsorbentcomposition is therefore dependent upon a number of interrelatedvariables. One such variable is the water content of the adsorbent whichis expressed herein in terms of the recognized Loss on Ignition (LOI)test. In the LOI test the volatile matter content of the zeoliticadsorbent is determined by the weight difference obtained before andafter drying a sample of the adsorbent at 500° C. under an inert gaspurge such as nitrogen for a period of time sufficient to achieve aconstant weight. It is preferred that the water content of the adsorbentresults in an LOI at 500° C. of less than 7.0% and preferably within therange of from 0 to 6.5 wt %.

The zeolite will ordinarily be present in the adsorbent particles assmall crystals in amounts ranging from about 75 to about 98 wt. % basedon volatile-free composition. Volatile-free compositions are generallydetermined after the adsorbent has been calcined at 900° C. in order todrive off all volatile matter. The remainder of the adsorbent willgenerally be the inorganic matrix present in intimate mixture with thesmall particles of the zeolite material. This matrix material may be anadjunct of the manufacturing process for the zeolite (for example, fromthe intentionally incomplete purification of the zeolite during itsmanufacture) or it may be added to relatively pure zeolite, but ineither case its usual purpose is as a binder to aid in forming oragglomerating the zeolite into the hard particles.

In the present invention the separation of a xylene isomer is effectedby passing a feed mixture comprising two or three isomers over a bed ofan adsorbent which selectively adsorbs the desired xylene whilepermitting other components of the feed stream to pass through theadsorption zone in an unchanged condition. The flow of the feed isstopped and the adsorption zone is then flushed to remove nonadsorbedmaterials surrounding the adsorbent. Thereafter the desired xylene isdesorbed from the adsorbent by passing a desorbent stream through theadsorbent bed, with the desorbent material comprising an aromatichydrocarbon described herein. The desorbent material is commonly alsoused to flush nonadsorbed materials from the void spaces around andwithin the adsorbent.

For purposes of this invention, various terms used herein are defined asfollows. A "feed mixture" is a mixture containing one or more extractcomponents and one or more raffinate components to be separated by theprocess. The term "feed stream" indicates a stream of a feed mixturewhich passes to the adsorbent used in the process. An "extractcomponent" is a compound or class of compounds that is more selectivelyadsorbed by the adsorbent while a "raffinate component" is a compound ortype of compound that is less selectively adsorbed. The term "desorbentmaterial" shall mean generally a material capable of desorbing anextract component from the adsorbent. The term "raffinate stream" or"raffinate output stream" means a stream in which a raffinate componentis removed from the adsorbent bed. The composition of the raffinatestream can vary from essentially 100% desorbent material to essentially100% raffinate components. The term "extract stream" or "extract outputstream" shall mean a stream in which an extract material which has beendesorbed by a desorbent material is removed from the adsorbent bed. Thecomposition of the extract stream, likewise, can vary from essentially100% desorbent material to essentially 100% extract components. At leastportions of the extract stream and the raffinate stream are passed toseparation means, typically fractional distillation columns, where atleast a portion of desorbent material is recovered to produce an extractproduct and a raffinate product. The terms "extract product" and"raffinate product" mean products produced by the process containing,respectively, an extract component and a raffinate component in higherconcentrations than those found in the extract stream and the raffinatestream. The term "rich" is intended to indicate a concentration of theindicated compound or class of compounds greater than 50 mole percent.

The rate of exchange of an extract component with the desorbent cangenerally be characterized by the width of the peak envelopes at halfintensity obtained from plotting the composition of various species inthe adsorption zone effluent during a pulse test versus time. Thenarrower the peak width, the faster the desorption rate. The rate ofexchange of various components can be expressed as "stage time" which iscalculated from the net retention volume and the half width peaks of thecomponents according to the formula in Principles of Adsorption andAdsorption Processes by Douglas M. Ruthven, published by John Wiley &Sons, 1984. The desorption rate can also be characterized by thedistance between the center of a tracer peak envelope and thedisappearance of an extract component which has just been desorbed. Thisdistance is the volume of desorbent pumped during this time interval.

Selectivity, (β), for an extract component with respect to a raffinatecomponent may be characterized by the ratio of the distance between thecenter of the extract component peak envelope and the tracer peakenvelope (or other reference point) to the corresponding distancebetween the center of the raffinate component peak envelope and thetracer peak envelope.

Relative selectivity can be expressed not only for one feed compound ascompared to another but can also be expressed between any feed mixturecomponent and the desorbent material. The selectivity, (β), as usedthroughout this specification is defined as the ratio of the twocomponents in the adsorbed phase divided by the ratio of the same twocomponents in the unabsorbed phase at equilibrium conditions. Relativeselectivity given by the equation: ##EQU1## where C and D are twocomponents of the feed represented in weight percent and the subscriptsA and U represent the adsorbed and unabsorbed phases, respectively. Theequilibrium conditions are determined when the feed passing over a bedof adsorbent does not change composition, in other words, when there isno net transfer of material occurring between the unabsorbed andadsorbed phases.

Where selectivity of two components approaches 1.0, there is nopreferential adsorption of one component by the adsorbent with respectto the other; they are both adsorbed to about the same degree withrespect to each other. As β becomes less than or greater than 1.0, thereis a preferential adsorption by the adsorbent for one component withrespect to the other. When comparing the selectivity by the adsorbent ofcomponent C over component D, a β larger than 1.0 indicates preferentialadsorption of component C within the adsorbent. A β less than 1.0 wouldindicate that component D is preferentially adsorbed leaving anunabsorbed phase richer in component C and an adsorbed phase richer incomponent D.

While separation of an extract component from a raffinate component istheoretically possible when the selectivity of the adsorbent for theextract component with respect to the raffinate component is not muchgreater than 1, it is preferred that such selectivity approach a valueof 2. Analogous to relative volatility in fractional distillation, thehigher the selectivity, the easier the adsorptive separation is toperform. Higher selectivities permit a smaller amount of adsorbent to beused.

The "speed" of the adsorption steps at various conditions or fordifferent adsorbent/desorbent combinations can be measured and comparedas stage times. Stage times are normally inversely correlated withtemperature. That is, as the temperature goes up, the stage times godown. A higher temperature is therefore normally desired since low stagetimes mean a smaller, less expensive plant is required to separate agiven quantity of feed material. On the other hand selectivity isnormally negatively impacted by higher temperatures. That is,selectivity normally decreases as the temperature goes up. In designinga commercial scale separation unit of this type, it is thereforenecessary to choose operating conditions based upon a balance ortrade-off of stage times versus selectivity.

An important characteristic of an adsorbent is the rate of exchange ofthe desorbent for the extract component of the feed mixture materialsor, in other words, the relative rate of desorption of the extractcomponent. This characteristic relates directly to the amount ofdesorbent material that must be employed in the process to recover theextract component from the adsorbent. Faster rates of exchange reducethe amount of desorbent material needed to remove the extract component,and therefore, permit a reduction in the operating cost of the process.With faster rates of exchange, less desorbent material has to be pumpedthrough the process and separated from the extract stream for reuse inthe process. Ideally, desorbent materials should have a selectivityequal to about 1 or slightly less than 1 with respect to all extractcomponents so that all of the extract components can be desorbed as aclass with reasonable flow rates of desorbent material, and so thatextract components can displace desorbent material in a subsequentadsorption step.

The preferred desorbent material used in an adsorptive separationprocess varies depending upon such factors as the type of operationemployed. In adsorptive separation processes, which are generallyoperated continuously at substantially constant pressures andtemperatures to insure liquid phase, the desorbent material must bejudiciously selected to satisfy many criteria. First, the desorbentmaterial should displace an extract component from the adsorbent withreasonable mass flow rates without itself being so strongly adsorbed asto unduly prevent an extract component from displacing the desorbentmaterial in a following adsorption cycle. Expressed in terms of theselectivity, it is preferred that the adsorbent be more selective forall of the extract components with respect to a raffinate component thanit is for the desorbent material with respect to a raffinate component.Secondly, desorbent materials must be compatible with the particularadsorbent and the particular feed mixture. More specifically, they mustnot reduce or destroy the capacity of the adsorbent or selectivity ofthe adsorbent for an extract component with respect to a raffinatecomponent. Additionally, desorbent materials should not chemically reactwith or cause a chemical reaction of either an extract component or araffinate component. Both the extract stream and the raffinate streamare typically removed from the adsorbent void volume in admixture withdesorbent material and any chemical reaction involving a desorbentmaterial and an extract component or a raffinate component or both wouldcomplicate or prevent product recovery. Finally, desorbent materialsshould be readily available and reasonable in cost, which is a problemwith some prior art "heavy" desorbents.

The highly preferred desorbent material of the subject invention is 1.4(or para) diisopropylbenzene which has a boiling point of 210° C.compared to the 184° C. boiling point of p-diethylbenzene. This allowseasy separation from the C₉ aromatics.

Feed mixtures which can be utilized in the process of this inventionpreferably comprise para-xylene, at least one other C₈ aromatic isomer,and may also contain one or more C₉ aromatics as impurities. Thus, thefeed mixtures to the process of this invention can contain quantities ofC₉ aromatics and may also contain quantities of straight or branchedchain paraffins, cycloparaffins, or olefinic material having boilingpoints relatively close to the desired xylene isomer. It is preferableto have these quantities at a minimum amount in order to preventcontamination of products from this process by materials which are notselectively adsorbed or separated by the adsorbent. Preferably, theabove-mentioned contaminants should be less than about 20% of the volumeof the feed mixture passed into the process.

Mixtures containing substantial quantities of para-xylene and other C₈aromatic isomers and C₉ aromatics generally are produced by catalyticnaphtha reforming and/aromatic hydrocarbon isomerization processes,processes which are well known in the refining and petrochemical arts.In catalytic naphtha reforming processes, a naphtha boiling range feedis contacted with a platinum and halogen-containing catalyst atseverities selected to produce an effluent containing C₈ aromaticisomers. Generally, the reformate is then fractionated to concentratethe C₈ aromatic isomers into a C₈ fraction which will also contain C₈nonaromatics and some C₉ aromatics. Many C₉ aromatics have boilingpoints in the range of 160°-170° C. and cannot be easily removed bydistillation from the prior art p-diethylbenzene desorbent.

In all of these catalytic routes either a xylene splitter column must beemployed to remove C₉ aromatics from C₈ aromatics or a heavy desorbentmust be used in order to obtain economic commercial scale operation,with the use of a heavy desorbent being the economically attractivechoice. I have discovered suitable desorbents which can be easilyseparated from the C₉ aromatics by fractionation and therefore do notrequire the large column and the quantity of utilities needed topretreat the feed, resulting in substantial cost savings.

Feed mixtures for the process of this invention may also be obtainedfrom isomerization and transalkylation processes. Xylene mixtures whichare deficient in one or more isomers can be isomerized, at isomerizationconditions, to produce an effluent containing C₈ aromatic isomers, e.g.,enriched in p-xylene, as well as C₈ non-aromatics and C₉ aromatics. TheC₉ aromatic content of isomerized xylene isomers can be as much as 1-2wt. % depending on isomerization conditions. Likewise, thetransalkylation of mixtures of C₇ and C₉ aromatics produces xyleneisomers.

Adsorption conditions include a temperature range of from about 20° toabout 250° C. with about 60° to about 200° C. being more preferred and apressure just sufficient to maintain liquid phase, which may be fromabout atmospheric to 600 psig. Desorption conditions will include thesame range of temperatures and pressure as used for adsorptionconditions.

A dynamic testing apparatus may be employed to test various adsorbentsand desorbent material with a particular feed mixture to measure theadsorbent characteristics of adsorptive capacity and exchange rate. Thispulse test apparatus consists of a helical adsorbent chamber ofapproximately 70 cc volume having inlet and outlet portions at oppositeends of the chamber. The tubular chamber is contained within atemperature control means and, in addition, pressure control equipmentis used to operate the chamber at a constant predetermined pressure.Quantitative and qualitative equipment, such as refractometers,polarimeters, chromatographs, etc., can be attached to the outlet lineof the chamber and used to analyze the effluent stream leaving theadsorbent chamber.

During a pulse test the following general procedure is used to obtaindata; e.g., selectivities, for various adsorbent/desorbent systems. Theadsorbent is filled to equilibrium with a particular desorbent bypassing the desorbent material through the adsorbent chamber. At aconvenient time, a pulse of feed containing known concentrations of atracer and of a particular extract component or of a raffinatecomponent, or both, all diluted in desorbent material is injected for aduration of several minutes. Desorbent flow is resumed, and the tracerand the extract and raffinate components are eluted as in a liquid-solidchromatographic operation. The effluent can be analyzed by on-streamchromatographic equipment and traces of the envelopes of correspondingcomponent peaks developed. Alternatively, effluent samples can becollected periodically and later analyzed separately by gaschromatography.

From information derived from the test, adsorbent/desorbent systemperformance can be rated in terms of void volume, retention volume foran extract or a raffinate component, and the rate of desorption of anextract component from the adsorbent and selectivity. Void volume is thenon-selective volume of the adsorbent, which is expressed by the amountof desorbent pumped during the interval from initial flow to the centerof the peak envelope of the tracer. The net retention volume of anextract or a raffinate component may be characterized by the distancebetween the center of the peak envelope (gross retention volume) of theextract or raffinate component and the center of the peak envelope (voidvolume) of the tracer component or some other known reference point. Itis expressed in terms of the volume in cubic centimeters of desorbentmaterial pumped during this time interval represented by the distancebetween the peak envelopes. The rate of exchange or desorption rate ofan extract component with the desorbent material can generally becharacterized by the width of the peak envelopes at half intensity. Thenarrower the peak width, the faster the desorption rate. The desorptionrate can also be characterized by the distance between the center of thetracer peak envelope and the disappearance of an extract component whichhas just been desorbed. This distance is again the volume of desorbentmaterial pumped during this time interval. Selectivity, β, can bedetermined by the ratio of the net retention volumes of the morestrongly adsorbed component to each of the other components.

A preferred embodiment of the invention may be characterized as aprocess for the adsorptive separation of p-xylene from a feed mixturecomprising C₉ aromatic hydrocarbons, p-xylene and at least one otherisomer of xylene which process comprises contacting said feed mixturewith an adsorbent comprising an X zeolite containing barium or bariumand potassium ions at exchangeable cationic sites at adsorptionconditions and effecting the selective adsorption of p-xylene by saidadsorbent and the production of a raffinate stream comprising said otherxylene isomers; and subsequently contacting said adsorbent with adesorbent chosen from the group consisting of diphenyl methane, 1,4diisopropylbenzene and 1,3 diisopropylbenzene at desorption conditionsto effect the removal of p-xylene from said adsorbent as an extractstream and recovering the p-xylene.

At first blush it may appear relatively simple to merely pick suitableheavy desorbents for this process using tables of physicalcharacteristics such as boiling points. It has been found, however, thatthis is not the case and that considerable judgment, experience andtesting is required to arrive at suitable desorbents. This is furthercomplicated by the sensitivity of the adsorption and desorption steps tonumerous variables including the hydration level of the sieve, thetemperature imposed on the system, the specific cations and level ofcation exchange of the zeolites. The data present below demonstrates theunpredictability in this area and the utility of diisopropylbenzenes asdesorbents for this process. The following non-limiting examples arealso presented to illustrate the process of the present invention andare not intended to unduly restrict the scope of the claims attachedhereto.

EXAMPLES

The experiments reported below summarize a number of pulse tests, usingthe apparatus as described above, to evaluate the ability of differentadsorbent/desorbent combinations to separate para-xylene (b.p. 138° C.)from the other xylene isomers and ethylbenzene (b.p.'s from 136°-145°C.) and from C₉ aromatics. The two adsorbents used were an X zeoliteexchanged with barium and potassium and a Y zeolite having potassiumloaded onto all its exchangeable sites. Both adsorbents comprised asimilar amorphous clay binder.

For each pulse test, the column was maintained at the indicatedtemperature and at a pressure of approximately 165 psig so as tomaintain liquid-phase operations. Gas chromatographic analysis equipmentwas attached to the column effluent stream in order to determine thecomposition of the effluent material at given time intervals. The feedmixture employed for each test was a mixture containing 20 vol. percentof each of the xylene isomers, ethylbenzene and normal nonane used as atracer. The desorbents were in some instances diluted with heptane toincrease the retention, time in order to make the results more precise.The compositions of the desorbents are given in Table 1. The operationstaking place for the test were as follows: The desorbent material wasrun continuously through the test apparatus at a rate of about 1.33 ccper minute. At some convenient time, the desorbent was stopped and thefeed mixture was injected into the column over a 3.8 minute interval.The desorbent stream was then resumed and continued to pass through theadsorbent column until all of the feed aromatics had been eluted fromthe column as determined by chromatographic analysis of the effluentmaterial leaving the adsorption column.

                  TABLE 1    ______________________________________    DESORBENT COMPOSITIONS    Boiling pt    °C.              vol. %    ______________________________________    A   293       30/70   phenyldecane/n-heptane    B   264.5     30/70   diphenylmethane/n-heptane    C   210       30/70   1,4-diisopropylbenzene/n-heptane    D   237       30/70   1,4-ditertiarybutylbenzene/n-heptane    E   203       100     1,3-diisopropylbenzene    F   216       30/70   1,3,5-triethylbenzene/n-heptane    G   205       30/70   5-tertiarybutyl-m-xylene/n-heptane    ______________________________________

Table 2 lists the net retention volume (NRV) for paraxylene, the stagetime and the selectivity, β, for ethylbenzene, orthoxylene andmetaxylene with respect to the reference p-xylene. The table also liststhe adsorbent used in each test, the hydration of the adsorbent by LOIand the temperature at which the test was run.

                                      TABLE 2    __________________________________________________________________________                         N.R.V.    LOI                  p-xy-           stage    @        Adsor-                 Temp                     Desor-                         lene                             p-x selectivity                                         time    Run No.         500° C.             bent                 (°C.)                     bent                         (ml.)                             EB  MX  OX  (sec.)    __________________________________________________________________________    8019-86         6.5 BaX 200 A   107 1.84                                 3.02                                     2.83                                         22.2    8019-87         0.5 KY  177 A       (no desorption of PX)    8026-99         0   BaX 200 B       (no separation)    8026-98         0   KY  200 B   10.5                             1.68                                 3.08                                     1.89                                         29.7    8273-4         0   KY  145 B   13.9                             1.73                                 2.50                                     2.16                                         27.6    8019-79         4.1 BaX 200 C   56.4                             2.28                                 3.45                                     3.34                                         19.8    8019-64         5.0 BaX 200 C   37.5                             1.90                                 2.58                                     2.46                                         18.7    8273-1         0   KY  200 C       (no desorption of PX)    8273-2         0   BaKX                 200 C       (no separation)    8026-95         0   BaX 185 D       (no desorption of all C.sub.8)    8026-94         0.2 BaKX                 185 D       (no desorption of all C.sub.8)    8026-96         0   KY  185 D   72  2.4 9.5 6.3 43.5    8019-60         4.1 BaX 200 E   52.7                             2.3 3.79                                     3.41                                         23.5    8026-100         0   KY  200 F   61.4                             2.33                                 7.58                                     5.23                                         40.8    8026-93         4.7 BaX 200 F   77.6                             1.3 3.0 2.32                                         43.9    8026-84         0.2 BaKX                 195 G       (no separation)    8026-89         5.6 BaX 200 G   73.2                             1.94                                 3.28                                     2.87                                         23.4    8026-89         0.1 KY  200 G   62.9                             2.37                                 4.37                                     3.82                                         15.4    8026-87         4.2 KY  200 G   47.0                             1.97                                 3.65                                     3.25                                         21.3    8026-88         4.7 KY  200 G   44.3                             1.97                                 3.59                                     2.23                                         28.2    8026-89         5.7 KY  200 G   47.1                             2.05                                 3.72                                     3.37                                         54.5    __________________________________________________________________________

An examination of the data given in Table 2 shows the presentlysurprising and unexpected results of how the interaction between themembers of the adsorbent/desorbent system affects xylene separationperformance. For instance, run 8273-1 using potassium exchangedY-zeolite 1,4 diisopropylbenzene shows no desorption of paraxyleneduring the desorption step while run 8026-96 using the same zeolite with1,4 ditertiarybutylbenzene shows good selectivities but somewhat higherthan desired N.R.V. and stage time. This illustrates a difference of onecarbon on the alkyl group can have a tremendous effect on systemperformance.

Another example of how system performance is dependent on the desorbentis the comparison of runs 8019-86, 8019-64 and 8026-89 which all use thesame barium exchanged X zeolite sieve at similar hydration levels andthe same temperature of 200° C. The data shows desorbent C (1,4diisopropylbenzene) to be much superior in terms of a lower retentionvolume and stage time compared to desorbent A (phenyldecane) anddesorbent G (5-tertiarybutyl-m-xylene). A comparison of runs 8026-99,8026-98 and 8273-4 shows the surprising result that desorbent B(diphenylmethane) gives good performance with a potassium Y sieve but noseparation with a barium X sieve.

What is claimed is:
 1. A process for the adsorptive separation of adesired xylene isomer from a feed mixture comprising C₉ aromatichydrocarbons and at least two isomers of xylene which process comprisescontacting said feed mixture with an adsorbent comprising a Y zeolitehaving one or more metal ions at exchangeable cationic sites atadsorption conditions and effecting the selective adsorption of thedesired xylene isomer by said adsorbent and the production of araffinate stream comprising a second xylene isomer; and subsequentlycontacting said adsorbent with a desorbent comprising diphenyl methaneat desorption conditions to effect the removal of the desired xyleneisomer from said adsorbent as an extract stream, and recovering thedesired xylene isomer.
 2. A process for the adsorptive separation ofp-xylene from a feed mixture comprising p-xylene and at least one otherisomer of xylene which process comprises contacting said feed mixturewith an adsorbent comprising an X or Y zeolite containing one or moremetal ions at exchangeable cationic sites at adsorption conditions andeffecting the selective adsorption of p-xylene by said adsorbent and theproduction of a raffinate stream comprising said other xylene isomers;and subsequently contacting said adsorbent with a desorbent comprisingdiphenyl methane at desorption conditions to effect the removal ofp-xylene from said adsorbent as an extract stream, and recovering thep-xylene.